This invention relates to an optical processing method and apparatus for enhancing radiographs, and the like.
Radiography is defined as the technique of producing a photographic image of an opaque specimen (typically a human body part) by transmitting a beam of electronic radiation (typically x-rays) through the specimen onto an adjacent photographic film. An image is produced on the film because the variations in thickness, density and chemical composition of the specimen serve to block or absorb some of the radiation energy, thereby causing the intensity of the radiation that does strike the photographic film (or other sensor) to be a function of the specimen through which the energy passed.
One problem in obtaining useful radiographs of humans is that for a body part being x-rayed, there typically is a need for different x-ray exposure levels over the length of that body part. Thus, one portion of the body part may only need a slight exposure to the x-ray to obtain the internal detail needed, whereas another portion may require much greater exposure to obtain the same detail. However, fixing some compromise level of exposure simply results in one portion being underexposed and another portion being overexposed. This is especially true for images having a wide range of exposure such as chest radiographs.
One approach for overcoming this problem is so-called unsharp masking which involves substracting a blurred copy of an original radiographic image from itself such that large-area exposure/density differences are selectively suppressed, while the contrast of fine details are hopefully preserved and in some cases enhanced. Two techniques which have been proposed for achieving unsharp masking include (a) blurred undersubtraction (BUS), see Kattan, K. R., "A Modified Blurred Undersubtraction Method (BUS) and Its Applications", Radiol, 1980, 134:782, and (b) Logetronography, see St. John, E. G., and Craig, D. R., "Logetronography", AJR. 1957; 78:123-133, and Street, J. N., MacIntosh, W. L., Mannack, A. W., and Gibson, R. C., "Optimizing Photographic Information Transferred By C.R.T.--A Technology and Applications Review", SPIE, 1984; 496:162-171. The BUS method involves photographic printing of the original film image or radiograph through a blurred negative of itself. A disadvantage of this is the requirement of a number of precise manipulations and positionings of film in a darkroom and is therefore quite time consuming. In Logetronography, a photographic copy is made of the original image by scanning the image with a cathode ray tube generated light beam, the intensity of which is modulated to compensate for regional density differences in the original image. The disadvantages of the Logetronography approach include inability to adjust the size and shape of the blurring function (light beam or source) used to generate the unshaped masking effect, and difficulty in eliminating light scattering and glare in the lenses and other glass components. Also, the long optical path present in currently used devices gives rise to large and cumbersome apparatus that is inconvenient in many standard-sized x-ray dark rooms. These disadvantages result in radiographs still containing blurred, unsharp images (although improved over the original radiograph).